Category: Amateur Radio

For a lot of experiments, knowledge of the Effective Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency, 1kHz is usually specified for SSB telephony receiver sensitivity measurement.

Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.

In brief, a white noise source is connected to the receiver input, Filter2 (nominal 2400Hz bandwidth soft response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.

Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.

I bought an inexpensive GPS antenna on eBay, the requirement was for one that operated from 3-5V to suit both of my GPSDOs. The antenna is mounted in an electrical junction box on conduit above the roof for reliable coverage.

The question was whether the active antenna with 5m of RG174 coax and SMA male connector at A$6 posted was any good.

To map its behaviour, it was attached to a Trimble Thunderbolt GPSDO and Lady Heather used to plot rx signal level over about 30h.

Above is the plot. The pattern is not quite symmetric as there are obstructions in play, in fact some of the dips in performance are explained by specific trees and the roof profile. There is a gap to the south at low elevations, GPS satellites don’t fly there (MEO inclined 55° from the equator) .

Overall, it reaches similar strength at the zenith as other antennas tried.

Overall evaluation, it seems to work ok though the coax is a bit rough.

Both cables are of similar size, ~10mm overall, stranded centre conductor and foil+braid outer conductor. The shield stranding is different and the foil is copper in the UF10, aluminium in the LMR400UF.

Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.

A recent long running thread on QRZ entitled “True balanced auto-tuner” was sure to tease out some pretty wooly thinking… the word “true” was enough to signal the outcome.

There are only three words in the title, we can dismiss “true” as a harbinger of wooly thinking, and though people will argue the toss on the appropriateness of the term “auto-tuner’, most people share an understanding of the meaning. “Balanced” is another problem altogether.

After thirty odd posts, there has been no definition or discussion of the term balanced, or its advantages or disadvantages.

One of the recommendations by several posters is the old is new again solution, the once popular link coupled tuner and the work of W5ZQ featured in one of those recommendations.

W5ZQ and WW8J

Topology

W5ZQ describes a tuner inspired by WW8J. W5ZQ extended the design and provides a writeup on optimising balance.

Above is W5ZQ’s partial circuit. In the article he describes and shows:

adjustment of the grounding point of the output tank; and

current meters which presumably attach to J2 and J3.

Key to analysis of the topology is that the centre of the output inductor is grounded. This results in the circuit tending towards equal but opposite phase voltages on the output terminals. Continue reading True balanced tuner

Given the lack of activity from credible stations on 40m, it seemed worth a checkout on 30m befor committing to the trial run, large download and data analysis.

So, I ran WSPR for a half hour just before 0000Z and observed the activity on WSPRnet map. I should note that my tx power was 0.1W and rx performance was impaired as there was a 20dB attenuator in line to achieve the tx power.

An experiment was conducted on 40m using WSPR to compare my own station transmit performance with others relatively close by.

The experiment was conducted around sunset on 01/08/2017, data was collected for the period 0600Z to 0900Z, sunset was at 07:17Z.

The experiment was unannounced as previous experience has been that if the WSPR community becomes aware of activity that does not accord with individual’s opinion of acceptable, the activity can be disrupted.

Data for analysis was fetched by downloading the archive which contained nearly 1,000,000 records for the day, and about 340,000 of those were 40m spots.

Factors shaping experiment design

The following is a discussion of various factors that weighed into experiment design.

Transmitters tend to cluster around the centre of the 200Hz WSPR band.

Above is a frequency distribution of tx frequency, and it is evident that the risk of interference is reduced by choosing a frequency near the upper or lower limit of the band segment. There was some activity just outside the designated band segment which might indicate care and competence of operators.

I was working recently on an antenna and a visitor was intrigued by a device I was using.

Above, the device is for tensioning a wire span, commonly called a come-along though that term gets applied to all manner for appliances for broadly similar purpose. A significant difference is that this was designed for smooth hard drawn copper wires, and has smooth flat jaws (70x6mm) and does not put a kink in the wire (as do most wire grips for more general fencing and FSWR use). This one has had the strap replaced, it came with a 1.25″ canvas reinforced rubber transmission belt which became hard over time. Continue reading Come-along from the past